Thioacetamide as a sulfur precursor for chalcopyrite thin film solar cells

Chalcopyrite CuInS2 (CIS) thin films have the potential to be used in low cost terrestrial photovoltaic applications. The band gap of CIS is around 1.5 eV [1] and it well matches with the solar spectrum. However, the power conversion efficiencies in CIS solar cells are presently limited to 13.2% [2]. CIS absorber layers can be produced in a simple two-stage process consisting of the physical vapor deposition of a Cu-In bilayer -- generally with Cu-rich composition - and its subsequent sulfurization under sulfur atmosphere made by either sulfur vapor or H2S. While sulfur vapor is a safer option than H2S for the sulfurization, gas-phase H2S is believed to provide much more reliable sulfur atmosphere and preferable reaction chemistry potentially making better crystalline structure and also removing residue oxygen in the layer -- it should be noted the best efficiency of 11.4% has been achieved with sulfur vapor so far though [3]. Previously, H2S and elemental sulphur were used as sources for sulphurization, but the stability and mobility of these sources are poor. This study has explored thioacetamide as a safer and more convenient source of sulfur for the sulfurization process. The thermal stability of thioacetamide has been investigated by differential scanning calorimetry and the decomposition temperature of thioacetamide was approximately 180°C. Upon decomposition, thioacetamide breaks down to H2S and acetonitrile, and H2S can be introduced into a sulfurization reactor. The chalcopyrite CIS phase was confirmed by X-ray diffraction and it exhibits polycrystalline thin films, with a strong diffraction signature at (112) direction. Scanning electron micrographs of CIS thin-films show the evidence of grain growth towards copper rich films. Thioacetamide allows the use of H2S during sulfurization process without having H2S gas bottles to start with minimizing safety issues associated with the transportation and the installation of H2S gas. Thioacetamide as a solid phase at room temperature also provides a very simple, low-cost sulfurization process configuration due to its stability.